二甲基甲醯胺為非揮發性溶液，其液滴在某些表面可表現非典型潤濕行
為，如: poly(methyl methacrylate)、經燃燒處理過的黃銅和藍寶石基材。類
似於水液滴擴散於完全潤濕性質基材，二甲基甲醯胺液滴展現一自發性擴
散，其擴散係數大於Tanner’s law。 液滴在擴散時，液滴的外圍厚度高於其
中心。與典型擴散不同，液滴在擴張到一定程度後會停止並開始向內收縮。
最終，液滴在五分鐘內會縮成球帽形狀並有著相當低的接觸角角度。有趣的
是，若在二甲基甲醯胺中添加界面活性劑，液滴會在擴張並收縮後表現一自
發性移動。其移動軌跡為隨機路線，擴散係數為0.005~0.01 mm2/s。有別於
大多數自發移動為反應性移動，二甲基甲醯胺液滴可重複跨越其經過的軌
跡。這種自發性移動可以被歸功於無接觸角遲滯的基材和Marangoni stress。
基於這些結果，我們提出了一種有關液滴的擴散、收縮和移動機制。

The nonvolatile N,N-dimethylformamide (DMF) droplet can display peculiar
wetting behavior on some substrates such as poly(methyl methacrylate), flametreated
brass, and sapphire. Similar to the expansion of a water drop on a total
wetting surface, the DMF droplet shows a spontaneous spreading initially but its
spreading dynamics is beyond Tanner’s law. The spreading droplet exhibits a
ridge near the rim whose height is greater than that of the center. Contrary to
typical spreading, the DMF droplet stops its outward expansion at some point and
begins inward contraction. Eventually, the droplet shrinks to a spherical cap with
a low contact angle within 5 min. It is interesting to find that upon addition of
surface-active agents, the droplet performs the self-propelled motion after
spreading-contraction. The trajectory is random and can be described as the
diffusive motion with the diffusivity ~0.005- ~0.01 mm2/s. Unlike self-propulsion
driven by reactive wetting, the DMF droplet can cross the trail left by itself. This
self-propulsion can be attributed to the effects of the hysteresis-free surface and
Marangoni stress. Based on those results, a mechanism explaining the contraction
and self-propelled droplet motion is proposed.

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